ABSTRACT Stroke is a leading cause of disability worldwide. Globally, there are 15 million stroke survivors each year who have significant neurological deficits including sensory and motor disability, leading to excessive socioeconomic burden. Cell therapy using primary MSCs has been actively explored by us and others as a therapeutic solution to this unmet medical need. However, since the therapeutic efficacy of MSCs has been shown to be dependent on intercellular communication between administered cells and brain parenchyma, we reasoned that the exosomes (nanoscale extracellular vesicles) that cells secrete to transmit this information could be used instead of and perhaps more efficiently than the cells themselves. Indeed, we were the first to report that systemic delivery of exosomes released by mesenchymal stromal cells (MSCs) to rats subjected to stroke or traumatic brain injury (TBI) substantially improves recovery of neurological function. A key rationale for using exosomes is that they obviate many of the risks associated with cell therapy because their use concentrates the active component of stem cells in a simpler compact non-replicative form. Importantly, they can cross the BBB allowing systemic delivery for treating the injured brain. Moreover, exosomes have low risk of immune rejection and are likely to be more cost-effective to produce and convenient to use and store and thus have potential for an ?off- the-shelf? treatment. However, there are significant roadblocks to translation of our preclinical results using MSC exosomes because of inherent limitations of MSCs for industrial scale production. MSCs are a poorly defined heterogeneous cell population with low proliferative capacity which can hinder batch consistency and limit scale- up for exosome production. Thus to overcome these roadblocks, we propose here to use exosomes derived from BioTime?s clonally pure PureStem progenitor cell lines, which have a high degree of homogeneity and are highly proliferative and regenerative because of their early embryonic origin. The goal of this phase I proposal is to conduct a feasibility study that would demonstrate neurological recovery in our rat stroke model using PureStem exosomes. We propose to identify 2 PureStem derived candidate exosomes based on angiogenic activity and micro RNA and protein content because increasing angiogenesis in the ischemic brain facilitates improvement of neurological function after stroke. The 2 lead candidates will be tested for efficacy measured by neurological function in our rat middle cerebral artery occlusion (MCAO) model. Evidence of feasibility using our well-established model to mimic human stroke and using exosomes from a scalable PureStem cell line will pave the way for further preclinical development and IND enabling studies in phase II.